1. Field of the Invention
This invention relates to the determination and display of signal level in decibels and more particularly to an apparatus and method for performing the same.
2. Description of the Prior Art
In many applications it is desirable to determine the level of a signal and display the same. For example, in testing a digital transmission line which carries a digital bit stream in a standard format consisting of 24 channels, a receiving test unit is connected to the line. The receiving unit selects one of the 24 channels and samples the PCM signal in that channel to determine among other things its amplitude level in decibels. That level is usually displayed along with other information about the signal such as its frequency and bit error rate. If a known PCM signal has been injected into that channel the information displayed by the receiving testing unit will then give an indication of the quality of signal transmission on that line.
As is well known in the art the signal level in decibels can be determined by performing the following steps:
(a) converting the compandored eight (8) bits in the PCM signal to a linear signal;
(b) calculating the square of the amplitude level of that signal;
(c) adding the squared amplitude level to the squared amplitude level for that signal that was calculated in the previous frame in accordance with steps a and b;
(d) repeating steps a, b, and c every 125 microseconds (the time for one frame) for a predetermined period of time, e.g. one second, so that a large number of samples of signal amplitude level are obtained;
(e) calculating the level at the end of that predetermined interval of time according to the formula ##EQU1## where S.sub.1 to S.sub.N are the samples of signal amplitude level and N is the number of samples;
(f) calculating the level in decibels according to the formula ##EQU2## where 1 mw is the reference level.
A processing unit such as a digital signal processor can and has been used to perform the steps described above. The level in decibels can then be displayed in any one of a number of known ways.
Alternatively and also as is well known in the art the level in decibels can be determined by the combination of a codec, RMS converter and A/D converter. The calculated level may then be displayed in decibels on a meter. The codec, RMS converter and A/D converter each introduce a conversion error and therefore the decibel level shown on the meter may not be the true level.
Over the past few years there has been increasing use of hand held battery powered equipment in the telecommunications industry to test digital transmission lines. Two examples of such hand held equipment are the T-TALKER (transmitting) and t-TAKER (receiving) units sold by the assignee of the present invention. Both of these units are powered by rechargeable batteries and weigh in the order of 19 ounces (540 grams). Such units are typically carried by a craftsperson for use in the field. In using such units, it may be desirable for the craftsperson to know the level in decibels of the signals being received at the receiving unit. Therefore it is necessary for that level to be calculated and displayed. It is also desirable that such a calculation and display be part of the hand held receiving unit without substantially changing the size of that unit.
As described above the level can be calculated by a digital signal processor. Such a processor does, however, use a number of rather expensive chip types. It also requires substantially more power for operation then is available in the hand held units described above. The batteries in the hand held units typically allow for about two to four hours of usage before they must be recharged. The level can also be calculated by the combination of converters described above. While such a combination and display can be packaged in its own portable unit, this means that a craftsperson must then use one unit for receiving signals and another unit for calculating and displaying levels. Of course, the receiving unit and the calculating and displaying unit could be put in a single package but the size of that single unit would be substantially larger then the size of the hand held receiving unit.
It is also desireable to not only calculate and display the amplitude level in decibels but also the frequency of the signal. The apparatus of the present invention is also capable of performing the calculation of signal frequency and displaying the same. A hand held receiving unit which incorporates the present invention can then calculate and display both amplitude level in decibels and signal frequency.